1. Field of the Invention
The invention relates to a method and apparatus for determining the oxygen content in gases, particularly combustion exhaust gases, uninfluenced by temperature variations, using at least one first electrochemical measuring cell which is heated to the operating temperature and is connected to a current source, through which a diffusion-limited electric current flows which depends on the oxygen content of the gas, and the magnitude of which is determined as a first electrical signal imaging the oxygen content of the gas.
2. Description of the Prior Art
In one known method of this kind, the electrochemical measuring cell is provided with a solid electrolyte which has electrodes on opposite surfaces. The electrode serving as the cathode is acted upon by the gas to be measured, wherein the oxygen content of the gas diffuses into the electrode and is reduced to oxygen ions which are transported through the solid electrolyte to the opposite electrode and are oxidized there to oxygen molecules. The electric current flowing with the oxygen ions through the solid electrolyte is a measure for the oxygen content of the gas. The rate at which the oxygen diffuses into the electrode is a function of the oxygen content of the gas to be measured so that the electric current flowing through the measuring cell depends on the diffusion rate of the oxygen into the electrode and therefore is called a diffusion-limited current. (U.S. Pat. No. 3,691,023).
In order that the oxygen content of a gas be measured quantitively with the above-mentioned method, the measuring cell should be kept at a constant operating temperature of at least about 400.degree. C., because the diffusion rate of gases depends heavily on temperature. This property of the measuring cell has a very detrimental effect in measuring operations, because it requires a very large amount of technical means to keep the desired temperature of the measuring cell constant to approximately .+-.2.degree.-5.degree. C.